The present invention relates to subterranean operations, and more particularly, to settable fluids comprising particle-size distribution-adjusting agents, and methods of using such settable fluids.
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings, such as casings and liners, are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
Set-delayed cement compositions are often utilized in circumstances where an operator finds it desirable to prepare a volume of a cement composition that remains in a pumpable state for a long period of time (e.g., for about two weeks or more), and that can be selectively activated to set into a hard mass at a desired time. For example, in circumstances where very large volumes of cement are utilized (such as in offshore platform grouting), the equipment required for mixing and pumping the requisite large volumes of cement composition may be very expensive, and may be difficult to assemble at the desired location. The storage of the requisite amount of dry cement prior to use may be another problem. As another example, the use of a set-delayed cement composition may also be desirable in circumstances where a relatively small volume of cement composition is used, such as a small construction job, for example, or a plugging and squeezing operation performed in the petroleum industry, for instance. In such circumstances, the cost to transport the cement composition to a job site, and to mix and pump it on location may be undesirable relative to the revenue generated from performing the cementing operation. Set-delayed cement compositions may be useful in circumstances such as those described above, as they can be prepared at a convenient location, then transported to and stored at a job site until use. At a desired time, the set-delayed cement composition may be mixed with a set activating agent; the resulting mixture may then be placed into a desired location (e.g., into a subterranean formation) and permitted to set therein. In some formulations, an excessive amount of set-activating agents have been injected into the set-delayed cement compositions, thereby “overactivating” the cement composition, after which a retarder is then added to the cement composition, in an attempt to fine-tune the eventual set time of the cement composition.
Operations involving conventional set-delayed cement compositions may encounter a number of difficulties. For example, the cement composition may thicken or gel with time, increasing the cement composition's viscosity, and thus impairing its pumpability. Another difficulty is that the activation process may be quite complicated, as exemplified by operations wherein the cement composition's set-time is first delayed until shortly before use, after which the cement composition is over-activated and again retarded.
Another problem that may occur with conventional set-delayed cement compositions is that the addition of set-activating agents may cause premature localized setting of the cement, e.g., localized regions within the bulk cement slurry wherein the set-activating agent becomes concentrated, thereby causing premature setting of a portion of the bulk cement. Such premature localized setting of the cement composition may be likely to occur when the cement composition is inadequately mixed. Premature localized setting of the cement composition may lead to pumping problems (e.g., hardened cement particles may damage pump impellers), and may also cause problems such as setting of the bulk cement while in storage tanks.
An additional difficulty posed by conventional set-delayed cement compositions is that the performance of the set-activating agents commonly used to selectively activate the cement compositions may be unpredictable. This may cause problems such as premature setting of the cement before placement (where the activating agent imparts an unexpectedly strong activating effect), or delayed setting of the cement after placement (where the activating agent imparts an unexpectedly weak activating effect). Both are undesirable.